This mode of printing therefore requires an air gap to be provided between the electrodes of the print head and the facing surface of the recording medium on which the latent image is formed. Thus, in order to form an image of electrostatic charge by means of a discharge through a gas, it is common practice not only to raise selected electrodes to a suitably high voltage, but also to provide a discharge gap of about 10 .mu.m as a function of Paschen's law curves for discharge through the gas in question, namely air at atmospheric pressure.
When the recording medium is constituted by a conductive paper having a dielectric layer which is a few micrometers thick of one of its surfaces, the natural roughness of the paper or the use of spacer elements embedded in the dielectric layer serve to obtain said discharge gap between the electrodes and the dielectric surface. However, when the recording medium has a substantially perfectly smooth surface, which is particularly true of dielectric film or of a metal drum covered with a dielectric layer (either of which may be used as an intermediate medium on which a latent image is produced for subsequent transfer after development onto a final medium), it is necessary to provide means in the vicinity of the print head itself for ensuring the presence of said discharge gap.
French patent No. 2 138 789 describes a device for generating charge images in a geometrically restricted space for the reproductive portions of an arrangement of electrodes which are situated opposite to a very high resistance dielectric layer or information carrier. In one of the embodiments described in said patent, an arrangement of electrodes is formed by distinct conductive styluses, each of which is embedded in an electrically insulating substance which has hollows at said arrangement of electrodes which are of the same diameter as the styluses, and in a variant, the styluses are symbol-shaped and are embedded in insulating material with symbol-shaped hollows whereby the bottom of the electrode arrangement does not reach the bottom of the insulating material so that the hollows constitute the necessary discharge gap. In another embodiment described therein, an arrangement of electrodes is constituted by a thin, highly conductive layer covering the walls of holes or openings made in an insulating material, or in a variant, by shaped holes made in a conductive material; in both cases, the discharge inside the holes or openings enables charge images to be created.
However, these electrostatic heads designed to incorporate the necessary discharge gap are not easy to manufacture on an industrial scale. Further, they do not appear to be capable of being used for high-speed printing at high resolution (up to 8 points per millimeter) as is obtained with multielectrode electrostatic heads (1728 electrodes) currently being used with paper covered in a dielectric layer and having surface irregularities, which heads use printed circuit technology to define the electrodes as the end portions of conductive tracks.
In these types of electrostatic print heads using printed circuit technology, the end faces of the conductive tracks are level with, or even project beyond, the edge of the substrate on which the tracks are printed. Such print heads are commonly made with two rows of electrodes, either by assembling two printed circuit carrying substrates back-to-back, or by placing back-to-back the two parts of a single substrate which has been folded or cut. Nonetheless, such heads will not operate with recording surfaces having a surface state which is too smooth.
Preferred embodiments of the present invention provide a print head in which an electric discharge gap is provided around the electrodes so that the head can operate with a recording medium having a smooth surface state, said head being made in printed circuit technology to define a linear arrangement of electrodes compatible with very high print performance.
Preferred embodiments of the present invention also keep the distance between each of the electrodes and the recording medium constant, independently of defects which would normally cause said distance to vary.